1. Field of Invention
The subject invention is generally related to a system for injecting continuous flexible conduit or pipe into a well against the pressure of the well and is specifically directed to an apparatus and method for pressurizing the exterior wall of the conduit as it is flexed to enter the well.
2. Discussion of the Prior Art
Apparatus for feeding continuous tubing or conduit into a well are well known. An example of such a system is shown and described in U.S. Pat. No. 4,091,867, entitled: "Flexible Conduit Injection System", issued to F. E. Shannon et al, on May 30, 1978. As there shown, the well typically includes a Christmas tree above the opening with a blow-out preventer at the top or outermost opening. A connector is attached to the blow-out preventer for receiving and feeding the flexible conduit into and out of the well casing. The conduit is fed from a spool located at a distance from the well. As the conduit is paid out from the spool, it is flexed around a guide member to direct the conduit into the connector and into the well.
The conduit is typically under well pressure during this operation. At the area where the conduit is flexed around the guide path member the wall stresses are greatly increased, particularly due to the pressure difference between the internal and external walls of the conduit. This added stress greatly contributes to reducing the life of the conduit.
A number of different methods and systems for bending or flexing the conduit have been developed in order to minimize the pressure induced stress at the points of maximum flex. Most have attempted to resolve the problem by minimizing the amount of flex or bending to which the conduit is subjected. U.S. Pat. No. 3,920,076, entitled: "Method for Inserting Flexible Pipe into Wells", issued to T. Laky on Nov. 18, 1975 utilizes a rotating capstan to maintain a tangential support of the pipe as it enters the well.
The aforementioned U.S. Pat. No. 4,091,867 provides for a closed guide member in fluid communication with the connector at the blow-out preventer and an injector drive means to maintain the conduit exterior under well pressure during the injection or withdrawal function. Rollers are provided in the interior of the guide means to minimize friction as the conduit is fed therethrough. While this apparatus includes the advantage of reducing the differential pressure on the conduit, and thereby minimizing the wall stress particularly at points of maximum stress, the device disclosed in the '867 patent has not gained widespread acceptance. In order to provide the external pressure equalization of the conduit at the point of maximum flex, it is necessary to provide a closed pressure chamber from the spool injector head to the blow-out preventer connector. This has proven to be a cumbersome apparatus with the improved results not worth the effort. Further, the rollers provided in the curved guide member proved to be both complicated and somewhat unreliable, with a substantial number of moving parts required to be maintained in good working order while the conduit is moved through the system.
While the advantages of maintaining reduced differential pressure on the interior and exterior walls of the conduit are well recognized as a viable means for reducing fatigue in the highest stressed areas, a reliable functional means for achieving this has not previously been available.